This invention relates to post-treatment of polyurea microcapsules with ammonia or amines to reduce the residual isocyanate level. Reduction of the residual isocyanate level, which results from the process in which the microcapsules are formed, permits flowable formulations of the microcapsules to be stored at pH values below 8 without the generation of carbon dioxide gas. Microcapsules are prepared by the process, in which polyurea capsule enclosures are formed around water-immiscible material by the interfacial polymerization of an organic isocyanate intermediate with water in the aqueous phase. The generation of carbon dioxide gas occurs when residual or unreacted isocyanate groups present after processing react with water in storage.
This invention also relates to post-treatment of polyurea microcapsules with ammonia to modify the permeability of the microcapsule wall. Modification of the permeability of the microcapsule wall is important to successful employment of the capsule and ultimately the material encapsulated therein. A high degree of cross-linking in the microcapsule wall can cause low permeability of the core material through the wall and affect the usefulness of the resulting capsule. A high degree of cross-linking can result from the use of di- and tri-functional monomers in the preparation of the polyurea microcapsules. With certain materials, a highly cross-linked wall structure is undesirable since it directly affects permeability of the core material to an adverse extent. To overcome the problem raised by the increased cross-linking, it has been found that the permeability of the resultant capsule wall can be modified by the post-treatment of the capsule wall with ammonia or an amine.
Post-addition of ammonia or amines to polyurea microcapsules reduces the residual isocyanate level in these capsules. Said addition modifies the permeability of the microcapsule wall to the core material and allows the formulation to be stored at pH values below about 8 without carbon dioxide generation.
More particularly, this invention relates to the production of improved polyurea microcapsule formulations containing the various core materials by the post-treatment of the formed polyurea microcapsules with ammonia or amines to reduce residual isocyanate levels. This improvement results in the ability to store flowable formulations of the microcapsules described herein at pH values below 8 without the generation of unacceptable carbon dioxide pressures in the sample containers. The process of this invention is directed to the production of improved microcapsule formulations, wherein the residual isocyanate level in the polyurea walls is reduced such that the generation of carbon dioxide is minimal, such process involving the post-treatment of said polyurea microcapsules with ammonia or amines.
More particularly, this invention relates to the production of improved polyurea microcapsule formulations containing various core materials by the post-treatment of the formed polyurea microcapsules with ammonia to modify the permeability of the capsule wall. This improvement results in a means for modifying the permeability of a polyurea microcapsule wall to achieve desired permeability. Post-treatment after the formation of the polyurea microcapsules with ammonia limits the cross-linking.
Reference is hereby made to Belgian Pat. No. 796,746, assigned to Stauffer Chemical Company, published Sept. 14, 1973. The aforementioned patent describes a method for encapsulating various water-immiscible materials employing an organic isocyanate intermediate to form a polyurea capsule enclosure around a water-immiscible material dispersed in an aqueous continuous phase.
Capsules of the nature and description disclosed hereinabove have a variety of uses, such as for containing dies, inks, chemical reagents, pharmaceuticals, flavoring materials, fertilizers, fungicides, bactericides, pesticides, such as herbicides, insecticides and the like, which substances can be dissolved, suspended or otherwise dispersed in or as the core material to be enclosed by the capsule. The material to be encapsulated can be employed in the initial dispersion at a temperature above its melting point, or dissolved or dispersed in suitable water-immiscible organic solvents. The nature of the water-immiscible material to be encapsulated can be organic or inorganic in nature. Once encapsulated, the liquid or other form is preserved until it is released by some means of instrumentality that breaks, crushes, melts, dissolves or otherwise removes the capsule skin, or until released by diffusion is effected under suitable conditions. An important specific aspect of this invention, together with other features and advantages contemplated by the invention, is the novel method for post-treatment of the poly-urea microcapsules formed by the above process with ammonia or amines to reduce the residual isocyanate level, thereby allowing storage of said microcapsules at pH values below 8.
Effective encapsulation by interfacial polymerization by an organic isocyanate intermediate can be accomplished in a process which utilizes two substantially immiscible liquids, one termed an aqueous phase and the other termed an organic phase, which comprises establishing a physical dispersion of the organic phase in the aqueous phase. Said organic phase contains the isocyanate intermediate for the polyurea capsule skin or enclosure. The interfacial polymerization, as it is termed, involves hydrolysis of an isocyanate monomer to form an amine which in turn reacts with another isocyanate monomer to form the polyurea enclosure. Capsules formed in this manner can range in size from about 0.5 microns to about 100 microns.
Isocyanates are capable of forming carbon dioxide gas by reaction with water in acidic solutions. In the above-described process, there remains unreacted isocyanate groups which in the presence of water are capable of forming carbon dioxide gas. When being stored, the microcapsules containing residual isocyanate, unless stored in the basic pH range, are capable of generating sufficient quantities of carbon dioxide to produce dangerous situations of pressure build-up and the like. Similarly, for certain materials, storage of the microcapsules containing the materials at pH ranges considered basic is not desirable since these microcapsules stored at pH values above about 7 will undergo hydrolytic decomposition. The post-treatment of the present invention is particularly useful for polyurea microcapsules of methyl parathion and the organic phosphate insecticideacaricide having as its active ingredient the compound N-(mercaptomethyl) phthalimide S-(O,O-dimethyl phosphorodithioate).